JPS6184323A - Manufacture of heat resisting austenitic stainless steel - Google Patents

Abstract

PURPOSE:To manufacture the titled stainless steel having improved mechanical properties, by executing the grain refining of crystal grains by subjecting a steel contg. specific percentage of C, Si, Mn, N, Ni, Cr, V and Mo to plastic deformation and heat treatment under specific conditions. CONSTITUTION:The steel consisting of, by weight, <=0.15% C, <=2.0% Si, <=3.5% Mn, 0.02-0.5% N, 8-18% Ni, 15-22% Cr, 0.01-1.0% V,. <=4% Mo, and essentially the balance Fe undergoes plastic deformation at >=20% draft. Then the material is heat treated at 1,050-1,150 deg.C to execute the refining of crystal grains. By this, the heat resisting austenitic stainless steel having improved yield strength can be manufactured.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a method for producing austenitic stainless steel with excellent mechanical properties, and in particular, to a method for producing austenitic stainless steel with fine grains and improved mechanical properties. Regarding manufacturing capacity no i.

[Technical background of the invention and its problems]

Austenitic stainless steel is often used in corrosive environments because of its excellent corrosion resistance. Furthermore, since the degree of temperature dependence of mechanical properties is smaller than that of ferritic steel, it is said that the service limit temperature can be lower than that of ferritic steel. Since the austenitic Suarez steel has a low yield strength, in actual use, the parts must have a thick wall structure to ensure a certain level of strength. As a result, the weight of the parts increases, making it difficult to transport and erect large parts, and at the same time, during heat treatment, the temperature distribution becomes uneven between the inner and outer surfaces of thick-walled parts.

Thermal fatigue is accelerated under conditions of repeated heating. Therefore, it is necessary to improve the mechanical properties of austenitic stainless steel and avoid thick structures as much as possible.

Generally, precipitation strengthening, solid solution strengthening, hot and cold working, etc. are used to improve mechanical properties such as strength and yield strength of austenitic stainless steel. However, these strengthening methods have the following limitations, and solid solution strengthening has not yet achieved sufficient strength. On the other hand, hot and cold working are effective in strengthening austenitic stainless steel because they make the grains finer and increase the strength through processing strain, but they are heated to high temperatures and are affected by heat during welding. Coarse crystal grains are formed in some areas, which causes a decrease in strength.

By the way, in order to improve thermal efficiency in thermal power plant plants that use oil or coal as fuel, there is a trend toward higher temperatures and higher pressures for vapor deposition, and martensitic stainless steel among turbine components lacks high-temperature strength. , a transition is being made to austenitic stainless steel, which has better high-temperature properties.

Further, in chemical plants and boilers, the conditions for use of austenitic stainless steel, which are used in high-temperature environments, are becoming harsher for the same reason.

Despite the changes in the environment in which steel is used, existing austenitic stainless steels lack mechanical properties, especially yield strength, and there is a need for austenitic stainless steels with even higher sieve yield strength. There is.

[Purpose of the invention]

The present invention was made in view of the above points, and is a method for manufacturing austenitic stainless steel with high strength without complicated heat treatment. It is an object of the present invention to provide a method for producing austenitic stainless steel having fine crystal grains that refines the crystal grains and increases yield strength by combining processing and simple heat treatment.

[Outline of the invention]

The present invention has a carbon content of 0.15% or less in terms of N amount percent, 2
．． 0% or less silicon, 3.0% or less manganese, 0.
02-0.3% nitrogen, 8-18% nickel, 15-0.
22% chromium, 0.01-1.0% vanadium, 4
Molybdenum of less than
Steel containing at least 1 f'ili of 0.5% titanium and 0.0005 to 0.01 boron is processed at a processing rate of 2.
1050~1150° after applying 0% or more plastic deformation
A method for producing lJ-stenitic stainless steel having fine crystal grains, which is heat treated at a temperature of 0 to refine the crystal grains.

Here, the reasons for limiting the composition of the austenitic stainless steel according to the present invention will be explained.

Carbon (C5 = carbon stabilizes the austenite phase.

It is more effective to strengthen stainless steel, but 0
．． Adding more than 15% will harm processability, so the upper limit is set at 0.15%, but from the perspective of processability, it is 0.12%.
The following is desirable. Furthermore, in practical terms, it is 0.03~
It is desirable to set it as 0.04-0.06 to 0.08%1%.

Silicon (St): Silicon acts as a deoxidizing agent during steel manufacturing and is a necessary element to improve the flowability of the metal during manufacturing, but since adding a large amount will impair toughness, the upper limit should be set at 2. .0chi. More preferably 0.1 to 1
．． 0%, especially [0.3 to 0.7%.

Manganese (Mn): Manganese acts as a deoxidizing agent in silicon-based MS steel and stabilizes the austenite phase as an austenite-forming element. 3.0%, preferably 0.2 to 2.4%, but practically 0.5 to 1.5% is desirable.

min: Nitrogen is necessary to stabilize the austenite phase and to improve the yield strength by forming a solid solution in the austenite phase. Also, along with vanadium, it has the effect of making crystal grains finer, so it must be contained in an amount of at least 0.02%, but if added in excess, it will not only create pinholes and blowholes, but also form nitrides at grain boundaries, reducing toughness. Therefore, the upper limit is set to O4β. Considering strength, blowholes, etc., it is desirable to set it at 0.08 to 0.35%, but considering industrial nitrogen control methods, it is desirable to set it at 0.08% to 0.35%.
．． It is desirable to set it as 11-0.2 inches.

Nickel (Ni): Nickel is an essential element to make the structure of stainless steel austenite.
% is required. However, if added in a large amount, the effect of making crystal grains finer will be weakened, and lξ will be expensive, so the upper limit is set as a percentage. In order to effectively refine the crystal grains, it is preferable to set the content to 8X15%, but in practice it is desirable to set the content to 8.5 to 13.5%, particularly 9.5 to 11.5%.

Chromium (Cr) Nichrome is required in an amount of 15% or more in order to increase strength at room and high temperatures, as well as improve corrosion resistance and oxidation resistance, but if added in large amounts, it forms a sigma phase and impairs toughness, so the upper limit should not be set. Section 22. Considering the balance with the Ni lid and the necessary amount of Cr when adding nitrogen, the content is preferably 16 to 19.5%, and more practically 16 to 18.5%.

Vanadium V): Vanadium is dissolved in the austenite phase to improve strength, and also acts with nitrogen to make crystal grains finer and improve yield strength.
% or more, but since adding too much causes segregation and impairs workability, the upper limit is set at 1.0. Considering mechanical properties and segregation, the content is preferably 0.03 to 0.5%, and more practically 0.05 to 0.35%.

Molybdenum (Mo): Molybdenum is dissolved in the austenite base material and has the effect of improving strength and corrosion resistance, but if added in large amounts, it may form a ferrite phase or cause segregation, impairing workability, so there is an upper limit. is set to 4%. More preferably, it is 0.5 to 3.5%, but in practice it is preferably 1.5 to 3.0%.

Niobium (Nb): Niobium needs to be contained at 0.01% in order to balance the amount of nitrogen and vanadium to make the crystal grains finer and to stabilize the fine crystal grains at high temperatures, but if added in large amounts, The upper limit is set at 1.0% since the effect is saturated and workability is impaired. More preferably, the ratio is 0.02 to 0.5, but for practical use, it is 0.02.
~015%.

Titanium (Ti): Like niobium, titanium needs to be contained at 0.02% in order to make the crystal grains finer and improve yield strength in balance with the amount of nitrogen and sodium, but if added in large amounts, it will cause segregation and reduce workability. Set the upper limit to 0 to avoid harm.
．． There will be Section 5. Considering the mechanical properties and segregation, 0,
It is desirable to set it to 0.02 to 0.3%, but in practice it is 0.02% to 0.3%.
02-0.15%.

Boron (B): Boron has the effect of strengthening grain boundaries, but if it is less than 0.0005, the effect is not sufficient and 0.01
Addition exceeding 0.0% will impair weldability, so the upper limit should be set at 0.0%.
1, preferably 0.001 to 0.008. Practically speaking, the content is preferably 0.003 to 0.007%.

In addition, after applying plastic deformation of austenitic stainless steel with the above composition at a processing rate of 20 or more, 1050 to 11
The reason for heat treatment at a temperature of 50°C will be explained next.

If the processing rate is less than 20, the grain size number (JISGO551) will be 5 even if heat treatment is performed for refinement.
Therefore, cold working and hot working of 20% or more are necessary. In addition, after processing of 20 or more, 1050 ~
The reason for heat treatment at 1150°C is that even if heat treated at a temperature lower than 1050°C, a processed structure remains and if heat treated at a temperature higher than 1150°C, the grain size number (JISGO551) of the crystal grain becomes less than 5 and it is no longer a fine grain. 11
The temperature should be in the range of 50°C.

〔Effect of the invention〕

According to the method for manufacturing austenitic stainless steel of the present invention, by combining the interaction of elements such as nitrogen and vanadium, hot and cold working, and simple heat treatment,
It is possible to improve yield strength by making crystal grains finer, and it is also possible to suppress the formation of coarse crystal grains that cause a decrease in strength in areas that are heated to high temperatures, such as heat shadows during welding.

[Embodiments of the invention]

After melting samples having the chemical compositions shown in Table 1 in a high frequency induction melting furnace, they were homogenized for 1100'020 hours and processed into 10 to 70 pieces by cold rolling. Next, the JIS grain size number of the sample refined at a temperature of 900 to 1200° C. for 2 hours was examined. Further, the sample of Example 1 was cold rolled to 10 to 70 inches after casting, and then refined at a temperature of 900 to 1200° C. for 2 hours. The grain size number of the sample was also examined. Comparative Example Sample 1 was commercially available 5US316 and was subjected to the same homogenization treatment, cold rolling, and refining treatment as the Example sample. These results are shown in Table 2.
It is shown in Table 3.

As shown in Tables 2 and 3, according to the method for manufacturing austenitic stainless steel according to the present invention, by combining processing and simple heat treatment, it has fine crystal grains that cannot be obtained with conventional alloys (Comparative Example 1). It can be seen that austenitic stainless steel can be manufactured. In this case, as shown in Table 2, if the refinement temperature is less than 1050°0, the processed structure will not disappear, and if it exceeds 1150°0, the grain size number will be 5.
This indicates that the crystal grains are smaller and larger.

Further, as shown in Table 3, when the processing rate is less than 20%, the grain size number becomes smaller than 5 as well.

Next, samples with compositions corresponding to Examples 1, 3.5 and Comparative Example 1 were subjected to f'4 analysis again, and after homogenization treatment at a time phase of 1100'020, they were processed by about 50% by cold rolling to 1050
After 0.02 hours of refining treatment, the JIS grain size number and tensile test were conducted to determine the 0.2% yield strength, which was designated as Examples 11, 12.13, and Comparative Example 3, respectively. The results are shown in Tables 4 and 5.

As shown in Table 5 below, the steel produced by the method for producing austenitic stainless copper according to the present invention is a conventional alloy of 5U.
It has finer crystal seed grains than S316 (Comparative Example 3), which can greatly improve mechanical properties, especially 0.2% proof stress.

As described above, the production method of the present invention can produce austenitic stainless steel that exhibits excellent mechanical properties, and can be used for parts that require high strength such as chemical plants and boilers. It can be said that this method is particularly suitable for manufacturing steel suitable for casings.

Agent: Patent Attorney Noriyuki Chika (1 other person)

Claims (1)

[Claims] 1) 0.15% or less carbon by weight percent, 2.0%
Silicon below, 3.5% below manganese, 0.02~
0.5% nitrogen, 8-18% nickel, 15-22%
of chromium, 0.01 to 1.0% of vanadium, 4% or less of molybdenum, and the remainder substantially iron, after plastic deformation of 20% or more at a processing rate of 1050 to 1150
A method for producing heat-resistant austenitic stainless steel, which is characterized by heat treatment at a temperature of °C to refine the crystal grains. 2) In claim 1, the grain size number (JIS G 0551) of the austenitic stainless steel is 5.
A method for producing heat-resistant austenitic stainless steel characterized by the above. 3) 0.15% or less carbon by weight percent, 2.0%
Silicon below, 3.5% below manganese, 0.02~
0.5% nitrogen, 8-18% nickel, 15-22%
chromium, 0.01-1.0% vanadium, less than 4% molybdenum, the balance being substantially iron and 0.01-1.0% vanadium.
0% niobium, 0.002-0.5% titanium, 0.0
After applying plastic deformation of 20% or more to steel containing at least one type of boron of 0.005 to 0.01%,
A method for producing heat-resistant austenitic stainless steel, which comprises heat-treating at a temperature of 50 to 1150°C to refine crystal grains. 4) In claim 3, the grain size number of crystal grains of austenitic stainless steel (JIS G 0551)
A method for producing heat-resistant austenitic stainless steel, characterized in that